As is well known, one form of xerographic reproduction device uses dry, particulate toner which is heat fused to paper to form a permanent image, usually black in color, on one or both sides of the paper.
A widely used heat fuser is a hot roll fuser. In this type of fuser the sheet of paper to be fused passed through the pressure nip formed by two rollers, usually cylindrical, which are in pressure contact. The quality of fusing produced by such a fuser is a function of temperature, time and pressure.
The pressure parameter is a function of the general construction of the hot roll fuser.
The time parameter is a function of the rotational speed of the fuser roll and the width of the fusing nip, this width being measured in the direction of paper movement. The width of the fusing nip is a function of the construction of the rolls. Hot roll fusers usable with the present invention may have any of the known construction, for example a soft heated roll and a hard unheated roll such as shown in U.S. Pat. No. 4,154,575, incorporated herein by reference.
The present invention is specifically related to a temperature control system for a hot roll fusing station, and while it will be explained in the environment of the hot roll fuser of U.S. Pat. No. 4,154,575, i.e. a fusing station having a soft hot roll and a hard, cold backup roll, it is not to be limited thereto.
The prior art has recognized the need to accurately control the temperature of a hot roll fusing station. In exemplary prior art a temperature control system includes an electrically energizable heater which is controlled by an electrical or electronic network which compares actual fuser temperature to a command set point temperature. The output of this network operates, in one manner or another, to energize the heater so as to cause the actual temperature to substantially achieve the set point temperature.
The means by which the fusing station's actual temperature has been sensed in the prior art includes a variety of specific constructions, and the selection of a specific construction to perform this function in the fuser temperature control system of the present invention is not critical thereto. In the preferred embodiments of the present invention the temperature sensing means is that shown in U.S. Pat. No. 3,809,855, incorporated herein by reference. However, the present invention is not to be limited thereto.
The use of a thermistor temperature sensing bridge circuit and a differential amplifier to control electrical energization of a heater is well known, as shown for example in U.S. Pat. No. 3,553,429.
In U.S. Pat. No. 3,705,289 an arrangement of this general type is shown in copying equipment where safety protection is provided should the resistance of the temperature varying resistor become too low (short circuit) or too high (open circuit).
U.S. Pat. No. 3,946,199 again shows this general arrangement in a copier. Here, the copier is maintained not-ready for use, after copier turn on, until an intermediate fuser temperature is sensed, whereupon the copier can be used as the fuser's temperature is maintained at a higher temperature. At the end of copier use, when the copier is turned off, a fan operates to cool the fuser until its temperature is sensed to be a temperature which is below the temperature at which the initial not-ready to ready transition occurred.
U.S. Pat. No. 3,985,433 also deals with maintaining a copying machine not-ready until a fuser enclosure heats up.
In U.S. Pat. No. 4,046,990, a hot roll fuser's silicone rubber covered heated roll has its temperature sensed by means of a temperature sensor 5 which is located in direct contact with an underlying metal core. An on-off or proportional controller 6 receives its input from the sensor, under the control of control logic, in response to certain information such as warm-up condition, copy start and/or copy stop control. The controller's output controls energization of a heater located within the heated roll. The fuser's temperature is maintained at an idling temperature setting, and is changed to a higher temperature upon the control logic indicating that copies will be forthcoming. In order to reduce the amplitude and duration of a fuser temperature overshoot, after a copy run state has been completed, it is said that the machine logic can be designed to cooperate with copy counters to cause the controller to control at the idle state temperature just prior to the end of the copy run.
In U.S. Pat. No. 4,145,599 a hot roll fuser temperature control system is suggested where four fuser temperatures are possible. The highest of these temperatures is that used for making copies. A lower temperature is a standby temperature which occurs when no copying operation is in effect, but the copier is ready for copying. In the event that a standby period is preceded by a long copy run, the fuser is maintained at a temperature which is lower than the above-mentioned standby temperature. This temperature is maintained for a time dependent upon the length of the copy run, whereupon the temperature returns to the higher standby temperature. The last of these four temperatures is the lowest of the four, and is the temperature below which the copier is maintained not-ready.